The concept of a portable telephone which uses a low or intermediate orbiting satellite as a communications satellite, has recently been proposed by various corporations. As the frequency bands for use in such communications, a frequency band of 1.6 GHz is assigned to communications from a ground portable telephone to a communications satellite, and a frequency band of 2.4 GHz is assigned to communications from the communications satellite to the ground portable telephone. The frequency band of 1.6 GHz is also assigned to a frequency band for use in bidirectional communications between ground stations and the communications satellite. A circularly polarized wave is commonly used in the communications in order to ensure the quality of a communications circuit.
An antenna has already been proposed as means for improving the quality of the communications circuit (as disclosed in Unexamined Japanese Patent Application No. Hei-7-183719). Specifically, a base conductor extends from a plane antenna in the direction opposite to an antenna element in order to improve the directivity of the antenna at a low elevation angle. FIG. 10 illustrates an example of a conventional antenna. In order to improve the directivity of the antenna at a low elevation angle, a microstrip plane antenna (MSA) 1 is comprised of a dielectric substrate 1c, a patched radiating element 1b provided on the dielectric substrate 1c, a ground conductor 1d attached to the bottom of the radiating element 1b, and a cylindrical ground conductor 1e downwardly extending from the base conductor 1d.
In a case where the conventional antenna receives an incoming circularly polarized wave from a satellite or sends the circularly polarized wave from a ground station to the satellite at a low elevation angle, the gain of the antenna and the axial ratio of the circularly polarized wave become too large, which in turn affects the quality of the communications circuit that is liable to variations in the positional relationship between the antenna of portable communications equipment and the antenna of the satellite. Thus, it has been difficult to maintain the sensitivity of communication of the antenna in every direction of the sky.
The present invention has been conceived in view of the aforementioned drawback in the art, and the object of which is to particularly improve the directivity and axial ratio of an antenna having a circularly polarized wave mode at a low elevation angle.
According to the present invention, the above-described object is accomplished by the structure disclosed in appended claims of the specification. More specifically, the present invention provides a composite antenna comprising:
a microstrip plane antenna (MSA) which possesses a circularly polarized wave mode and is made up of a conductive plate serving as a common base conductor, a dielectric layer provided on the conductor plate, and a patched radiating element provided parallel to the conductor plate with the dielectric layer between them; PA1 a linear radiating element which is helically wrapped in a substantially coaxial relationship with respect to the microstrip plane antenna and is provided below the conductor plate; and PA1 the upper ends of the helically coiled linear radiating element being electrically connected to the conductor plate, thereby forming a helical antenna. The helical antenna may be connected to the conductor plate by DC or capacitive coupling.
The directivity of a radiation pattern at a high elevation angle greatly depends on a plane portion of the patched radiating element of the MSA. In contrast, the directivity of the radiation pattern at a low elevation angle greatly depends on the helical antenna and the electric field developed between the periphery of the patched radiating element of the MSA and the base conductor.
If the base conductor of the MSA is downwardly extended as are the base conductor of the conventional antenna, the antenna has a high sensitivity with regard to a polarized wave in the axial direction of the antenna (i.e., a vertically polarized wave) but a low sensitivity with regard to a horizontally polarized wave.
According to the present invention, the sensitivity of the antenna with regard to the horizontally polarized wave is improved by electrically coupling the helical antenna to the conductor of the MSA in the way as previously described. The helical antenna contributes to improvements in the sensitivity of the antenna with regard to the horizontally polarized wave, due to horizontal components made of high frequency currents which flow through the helical antenna. The line width, length, the number of turns of the helical element, and the pitch with which the helical element is coiled, may be designed according to a satellite communications system as required.